Electrochemical cells including a nonaqueous electrolyte are substantially free of water. The cell electrode materials and electrolyte are carefully manufactured, dried and stored prior to cell manufacturing to maintain the amount of water in those components at typically no more than tens or hundreds of parts per million. Manufacturing processes in which cell internal components are exposed to the air are generally performed in a dry box or a dry room. These measures are necessary because of the high reactivity of one or more of the cell ingredients with water. Organic solvents or solutions are often used as electrolytes in nonaqueous cells. Examples of nonaqueous cells that contain such organic solvents include lithium and lithium ion cells, although other types of nonaqueous cells, containing other materials that are highly reactive with water, are known.
Batteries containing nonaqueous cells are becoming increasingly popular as power sources for electronic devices. Though they are often more costly than common aqueous cells, nonaqueous cells can have many advantages because of the natures of materials used. These advantages include high energy density, high capacity at low temperatures, low weight and excellent shelf life over a broad range of temperatures. Many nonaqueous cells also have high electrode interfacial surface area designs that make them especially well suited for high power (including high current and low resistance) discharge, and the general trend in power requirements for electronic devices has been toward higher and higher power. Some of the types of devices for which high capacity on high power discharge is particularly important include photoflash devices (flash units and cameras with internal flash capability), digital still cameras, video cameras, personal digital assistant devices and portable computers.
The electrolytes comprising organic solvents or solutions utilized in nonaqueous cells in some embodiments have relatively low boiling points and thus relatively high vapor pressures within the cell at normal operating and/or storage temperatures. It is, therefore, a requirement for the cell to be sealed properly to resist leakage of liquid electrolyte as well as resist transmission of electrolyte vapor or gas from within the cell to a location outside the cell.
A wide variety of cell designs have been used for nonaqueous cells. The type of design is dependent in part on the size of the cell, the type of electrode and electrolyte materials used in the cell and the power requirements of the devices to be powered by the cell. Because the cathode/electrolyte materials are so reactive, the designs for large liquid cathode lithium cells (e.g., lithium-sulfur dioxide (Li/SO2) and lithium-thionyl chloride (Li/SOCl2)) often have housings in which metal components are hermetically welded, and glass seals are used to seal metal components that must be electrically insulated and to seal small apertures in the housings. These types of housings tend to be expensive due to the materials and the manufacturing processes and equipment required.
Other means can be used to seal the cells. Because of the relatively low cost and ease of manufacture, it can be desirable to use thermoplastic seal members between rigid housing components. For example, a thermoplastic gasket or grommet can be compressed between the inside top edge of the cell container (e.g., a steel can) and the periphery of the cell cover closing the open top of the can, forming a seal to keep the electrolyte within the cell housing and to keep water out.
A thermoplastic seal member can also be used to seal an aperture in the cell housing. For example, the thermoplastic seal member may be in the form of a plug sealing a small hole in the cell cover. Electrolyte may be dispensed into the cell after the cover has been assembled to the can. In another example, the plug may be a rigid material, such as a glass or metal ball, with a thermoplastic seal member in the form of a bushing between the inner surface of the aperture and the ball. In these examples, the thermoplastic plug or the ball and bushing may also function as a pressure relief vent for the cell.
Cylindrical lithium cell designs have been used for Li/FeS2 and other lithium cell types that include two thermoplastic seal members—a gasket sealing a cover in the open end of the can and a bushing sealing an aperture in the cell cover. Both thermoplastic seal members provide a compressive seal. Since the can and cover are electrically connected to opposite electrodes within the cell, the gasket also provides the necessary electrical insulation. The bushing and a vent ball comprise a pressure relief vent for the cell. When the internal cell pressure exceeds a predetermined abnormally high level, the vent ball, or the ball and bushing, are forced out of the cover, leaving an opening through which pressure is released. Cells sealed with both a gasket between the can and cover and a pressure relief vent comprising a bushing and vent plug disposed in an aperture in the cell cover are disclosed in U.S. Pat. No. 4,329,405 (issued May 11, 1982), U.S. Pat. No. 4,437,231 (issued Mar. 20, 1984), U.S. Pat. No. 4,529,673 (issued Jul. 16, 1985), U.S. Pat. No. 4,592,970 (issued Jun. 3, 1986), U.S. Pat. No. 4,927,720 (issued May 22, 1990), U.S. Pat. No. 4,931,368 (issued Jun. 5, 1990) and U.S. Pat. No. 5,015,542 (issued May 14, 1991), the entire disclosures of which are incorporated herein by reference.
Thermoplastic seal members are also used in other types of cells, including aqueous electrolyte cells such as common consumer type aqueous zinc-manganese dioxide (Zn/MnO2), nickel-cadmium (Ni/Cd) and nickel-metal hydride (NiMH) cells.
For any cell type, the seal member material and design must be such that a suitable seal is maintained for an acceptable period of time and under the temperature conditions that the cell is expected to withstand during transportation, storage and use. Common characteristics of a good seal member include stability of the material in the internal cell and external environments, impermeability to the liquids and gases that are to be sealed within or outside the cell, and the formation and maintenance of a complete seal path (i.e., with no voids or gaps) at each seal interface.
For thermoplastic seal members which form a compressive seal, the seal member must be sufficiently compressed to achieve a good seal, and sufficient compression must be maintained for the desired time. Thermoplastic materials under compressive stress tend to move to relieve that stress. This is referred to as stress relaxation or cold flow of the material. Thermoplastic materials tend to stress relax more at higher temperatures, thereby reducing the time that sufficient compression can be maintained. Temperature also affects the compression of thermoplastic seal members in another way. Different materials will expand and contract by different amounts in response to increases and decreases, respectively, in ambient temperature. In a cell with a thermoplastic seal member forming a compressive seal between more rigid components (e.g., a metal can and a metal cover), it is generally desirable for the gasket and rigid components being sealed to expand at close to the same rate in order to maintain sufficient gasket compression over the greatest temperature range possible.
Thermoplastic materials and seal designs suitable for nonaqueous cell seal members are more limited than for aqueous cell seal members. The seal members must have a higher degree of impermeability to water because active materials in the cell are very reactive therewith, and some common materials for aqueous cell seal members are not suitable. Nonaqueous cell seal members must also have a low vapor transmission rate for the electrolyte solvents. Since the vapor transmission rates of thermoplastic materials are generally dependent in part upon the vapor pressure of the solvent, low vapor transmission rates are generally more difficult to achieve for nonaqueous cells whose electrolytes contain ethers or other organic solvents or compounds with low boiling points. The greater the ratio of the effective area of the seal member exposed to the internal volume of the cell, the more important the electrolyte solvent and water transmission rates.
Polypropylene is commonly used as a material for lithium cell (e.g., Li/MnO2 and Li/FeS2) gasket seal members. Gaskets have been made with other thermoplastic materials for the purpose of improving the ability of the cell to withstand high temperatures than with polypropylene.
U.S. Pat. No. 4,282,293 discloses a seal for alkaline cells comprising a gasket having a coated layer of a polyamide, an epoxy resin, asphalt or a cured epoxy-polyamide resin and a film of a substituted organosilane disposed and compressed between the interface of the cell's cover and the coated gasket of the cell thereby reportedly providing a fluid-tight seal therebetween.
Reissued U.S. Pat. No. RE 35,746 discloses a battery package for a thin battery including a flexible base film that covers and encloses the battery and a flexible layer of an inorganic material such as silicon nitride, aluminum nitride or aluminum oxide deposited on the base film to reportedly encapsulate and seal the battery. The base film is formed of a flexible polymer material such as polyester that may be attached to the battery using a heat activated adhesive. The layer of inorganic material is deposited on the base film utilizing a low temperature CVD deposition process either before or after the base film is attached to the battery.
U.S. Patent Application Publication No. 2005/0079404 discloses an electrochemical battery cell with an aperture in the container or cell cover having the aperture sealed by an improved thermoplastic sealing member, which forms at least a part of the cell's pressure relief vent and is made from a material comprising a thermoplastic resin and more than 10 weight percent of a thermal-stabilizing filler, to provide an effective seal and a reliable pressure relief vent over a broad temperature range.
U.S. Patent Application Publication No. 2005/0079413 discloses an electrochemical battery cell with an improved thermoplastic sealing member. The seal member is made from a thermoplastic resin comprising polyphthalamide or impact modified polyphenylene sulfide. The seal member reportedly provides an effective seal vent over a broad temperature range and has a low electrolyte vapor transmission rate.
Japanese Publication No. 58-087755 relates to reportedly preventing electrolyte moving up to the outer surface of the battery through the surface of a negative can due to electric capillarity, by the sealing effect of a magnetic field, with the entire contact surfaces of an insulating gasket, a positive can and the negative can, without using any permanent magnetic ring. The magnetic fluid used in the example is a colloidal solution prepared by dispersing magnetite particles which have diameters of 100-200 Angstrom and are coated with oleic acid in carbon fluoride solvent by use of a nonionic surfactant; here, an electrolyte-resistant solvent is selectively used. Since the insulating gasket also serves as a magnet, the thin layer of the magnetic fluid reportedly intensely adheres to the surface of the gasket due to its magnetic force, which prevents any fluid from flowing out of the battery. Thus, a liquid sealing between the positive cap and the negative can is reportedly enabled.
Japanese Publication No. 60-182656 relates to an insulating film of metal oxide, such as alumina or chromate, formed on the surface, which is to be in contact with a gasket of an anode can by vapor deposition or chemical treatment process. Silicon dioxide particles having a particle size of 10-150 Angstrom (silica sol) are embedded into 100-300 Angstrom defects or micro pores of the insulating film. When chromate film is formed by chemical treatment, silica sol is added into chromate treatment solution, and silica particles are reportedly embedded into a chromate film when chromate is deposited. For example, the anode can with insulating film is subjected to heat treatment at 150° C. or more (400-600° C.). By this heat treatment, silica particles reportedly become insoluble in water and alkali resistant.
Japanese Publication No. 09-035694 relates to a laminated body prepared by stacking about a 50μ thick modified polyethylene film, a metal laminate film obtained by piling about 40μ thick resin layers on each side of an about 10μ thick metal plate, and an about 10μ thick resin layer. An electrode group is inserted into a cylindrical can body with the bottom having a square cross section, an electrolyte is poured, a sealing plate and a gasket are fit to an opening end of the can body, the opening end is caulked, the gasket and the sealing plate are interposed between the opening end and a projection strip, the opening end is sealed to form a caulked part. The laminated body is arranged so as to cover the caulked part and the sealing plate, the laminated body is pushed to a hot plate to melt the film, and the laminated body is fused to the portion over from the opening end of the cart body to the sealing plate.
Japanese Publication No. 2002-198019 relates to a small-sized lithium cell with reportedly high accuracy, excellent air tightness, and good workability at manufacture, without slippage of a gasket when mounting is obtained by laminating and integrally molding a gasket made of a synthetic polymer resin film and a positive electrode metal case. A biodegradable synthetic polymer resin film, especially a polyvinyl alcohol film, is recommended as a synthetic polymer resin film. By using this film, the danger of environmental pollution is reportedly eliminated because the film is quickly decomposed when discarded as a used cell.
Thermoplastic seal members, such as polypropylene seal members can have high solvent vapor transmission rates. The problem of reducing the rate of transmission of electrolyte vapor or gas through the seal member is generally greater at higher temperatures and with more volatile organic solvents with relatively low boiling points. Therefore, it would be desirable to have an electrochemical battery cell with improved seal characteristics, especially wherein vapor transmission of electrolyte through one or more seal members is reduced.